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  设计模式之责任链模式
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  <p>责任链模式</p>
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<h2 id="责任链模式"><a href="#责任链模式" class="headerlink" title="责任链模式"></a>责任链模式</h2><p>责任链模式（Chain of Responsibility Pattern）是将链中每一个节点看作是一个对象，每个节点处理的请求均不同，且内部自动维护一个下一节点对象。当一个请求从链式的首端发出时，会沿着链的路径依次传递给每一个书点对象，直至有对象处理这个请求为止，属于行为型模式。</p>
<p>通用UML类图</p>
<p><img src="https://tu.dxysun.com/image-20210424183802202-20210424183805.png" alt="image-20210424183802202"></p>
<p>从UML类图中，可以看到，责任链模式主要包含两种角色：</p>
<p>抽象处理者（Handler）：定义一个请求处理的方法，并维护一个下一个处理节点Handler对象的引用；</p>
<p>具体处理者（ConcreteHandler）：对请求进行处理，如果不感兴趣，则进行转发。</p>
<p>责任链模式的本质是解耦请求与处理，让请求在处理链中能进行传递与被处理；</p>
<p>理解责任链模式应当理解的是其模式（道）而不是其具体实现（术），责任链模式的独到之处是其将节点处理者组合成了链式结构，并允许节点自身决定是否进行请求处理或转发，相当于让请求流动了起来。</p>
<h3 id="责任链模式的应用场景"><a href="#责任链模式的应用场景" class="headerlink" title="责任链模式的应用场景"></a>责任链模式的应用场景</h3><p>在日常生活中责任链模式还是比较常见的，我们平时工作处理一些事务，往往是各部门协同合作的完成某一个任务。而每个部门都有各自的职责，因此，很多时候事情完成一半，便会转交给下一个部门直到所有部门都过一遍之后事情才能完成。还有我们平时俗话说的过五关、斩六将其实也是一种责任链。</p>
<p>责任链模式主要是解耦了请求与处理，客户只需将请求发送到链上即可，无需关心请求的具体内容和处理细节，请求会自动进行传递直至有节点对象进行处理。适用于以下应用场景</p>
<p>1、多个对象可以处理同一请求，但具体由哪个对象处理则在运行时动态决定；</p>
<p>2、在不明确指定接收者的情况下，向多个对象中的一个提交一个请求；</p>
<p>3、可动态指定一组对象处理请求。</p>
<h3 id="通用代码"><a href="#通用代码" class="headerlink" title="通用代码"></a>通用代码</h3><p>抽象处理者（Handler）</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">abstract</span> <span class="class"><span class="keyword">class</span> <span class="title">Handler</span></span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line"></span><br><span class="line">	<span class="keyword">protected</span> Handler nextHandler;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setNextHandler</span><span class="params">(Handler nextHandler)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		<span class="keyword">this</span>.nextHandler = nextHandler;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">abstract</span> <span class="keyword">void</span> <span class="title">handlerRequest</span><span class="params">(String req)</span></span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>具体处理者（ConcreteHandler）</p>
<p>ConcreteHandlerA</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ConcreteHandlerA</span> <span class="keyword">extends</span> <span class="title">Handler</span></span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">void</span> <span class="title">handlerRequest</span><span class="params">(String req)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		System.out.println(<span class="string">"ConcreteHandlerA handlerRequest "</span> + req);</span><br><span class="line">		<span class="comment">// 可做一些处理，决定是否往下传递</span></span><br><span class="line">		<span class="keyword">if</span> (nextHandler != <span class="keyword">null</span>)</span><br><span class="line">		&#123;</span><br><span class="line">			nextHandler.handlerRequest(req);</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>ConcreteHandlerB</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ConcreteHandlerB</span> <span class="keyword">extends</span> <span class="title">Handler</span></span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">void</span> <span class="title">handlerRequest</span><span class="params">(String req)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		System.out.println(<span class="string">"ConcreteHandlerB handlerRequest "</span> + req);</span><br><span class="line">		<span class="comment">// 可做一些处理，决定是否往下传递</span></span><br><span class="line">		<span class="keyword">if</span> (nextHandler != <span class="keyword">null</span>)</span><br><span class="line">		&#123;</span><br><span class="line">			nextHandler.handlerRequest(req);</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>测试代码</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Main</span></span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		ConcreteHandlerA concreteHandlerA = <span class="keyword">new</span> ConcreteHandlerA();</span><br><span class="line">		ConcreteHandlerB concreteHandlerB = <span class="keyword">new</span> ConcreteHandlerB();</span><br><span class="line">		concreteHandlerA.setNextHandler(concreteHandlerB);</span><br><span class="line">		concreteHandlerA.handlerRequest(<span class="string">"req"</span>);</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>输出</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">ConcreteHandlerA handlerRequest req</span><br><span class="line">ConcreteHandlerB handlerRequest req</span><br></pre></td></tr></table></figure>



<h3 id="责任链模式和建造者模式结合使用"><a href="#责任链模式和建造者模式结合使用" class="headerlink" title="责任链模式和建造者模式结合使用"></a>责任链模式和建造者模式结合使用</h3><p>因为责任链模式具备链式结构，在上述代码中负责组装链式结构的角色是ConcreteHandler，当链式结构较长时，ConcreteHandler的工作会非常繁琐，并且ConcreteHandler代码相对臃肿，且后续更改处理者或消息类型时，都必须在ConcreteHandler中进行修改，不符合开闭原则。产生这些问题的原因就是因为链式结构的组装过于复杂，而对于复杂结构的创建，很自然我们就会想到建造者模式，使用建造者模式，我们完全可以对ConcreteHandler指定的处理节点对象进行自动链式组装，客户只需指定处理节点对象，其他任何事情都无需关心，并且客户指定的处理节点对象顺序不同，构造出来的链式结构也随之不同。我们来改造一下，修改Handler的代码：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">abstract</span> <span class="class"><span class="keyword">class</span> <span class="title">Handler</span></span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line"></span><br><span class="line">	<span class="keyword">protected</span> Handler nextHandler;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">next</span><span class="params">(Handler nextHandler)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		<span class="keyword">this</span>.nextHandler = nextHandler;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">abstract</span> <span class="keyword">void</span> <span class="title">handlerRequest</span><span class="params">(String req)</span></span>;</span><br><span class="line"></span><br><span class="line">	<span class="keyword">public</span> <span class="keyword">static</span> <span class="class"><span class="keyword">class</span> <span class="title">Builder</span></span></span><br><span class="line"><span class="class">	</span>&#123;</span><br><span class="line">		<span class="keyword">private</span> Handler head;</span><br><span class="line">		<span class="keyword">private</span> Handler tail;</span><br><span class="line"></span><br><span class="line">		<span class="function"><span class="keyword">public</span> Builder <span class="title">addHandler</span><span class="params">(Handler handler)</span></span></span><br><span class="line"><span class="function">		</span>&#123;</span><br><span class="line">			<span class="keyword">if</span> (<span class="keyword">this</span>.head == <span class="keyword">null</span>)</span><br><span class="line">			&#123;</span><br><span class="line">				<span class="keyword">this</span>.head = <span class="keyword">this</span>.tail = handler;</span><br><span class="line">				<span class="keyword">return</span> <span class="keyword">this</span>;</span><br><span class="line">			&#125;</span><br><span class="line">			<span class="keyword">this</span>.tail.next(handler);</span><br><span class="line">			<span class="keyword">this</span>.tail = handler;</span><br><span class="line">			<span class="keyword">return</span> <span class="keyword">this</span>;</span><br><span class="line">		&#125;</span><br><span class="line"></span><br><span class="line">		<span class="function"><span class="keyword">public</span> Handler <span class="title">build</span><span class="params">()</span></span></span><br><span class="line"><span class="function">		</span>&#123;</span><br><span class="line">			<span class="keyword">return</span> <span class="keyword">this</span>.head;</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>测试代码</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Main</span></span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line"></span><br><span class="line">		ConcreteHandlerA concreteHandlerA = <span class="keyword">new</span> ConcreteHandlerA();</span><br><span class="line">		ConcreteHandlerB concreteHandlerB = <span class="keyword">new</span> ConcreteHandlerB();</span><br><span class="line">		Handler.Builder builder = <span class="keyword">new</span> Handler.Builder();</span><br><span class="line">		builder.addHandler(concreteHandlerA)</span><br><span class="line">				.addHandler(concreteHandlerB);</span><br><span class="line"></span><br><span class="line">		builder.build().handlerRequest(<span class="string">"req"</span>);</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>输出</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">ConcreteHandlerA handlerRequest req</span><br><span class="line">ConcreteHandlerB handlerRequest req</span><br></pre></td></tr></table></figure>



<h2 id="责任链模式在源码中的体现"><a href="#责任链模式在源码中的体现" class="headerlink" title="责任链模式在源码中的体现"></a>责任链模式在源码中的体现</h2><p>首先我们来看责任链模式在JDK中的应用，来看一个J2EE标准中非常常见的Filter类：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Filter</span> </span>&#123;</span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">init</span><span class="params">(FilterConfig filterConfig)</span> <span class="keyword">throws</span> ServletException</span>;</span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">doFilter</span><span class="params">(ServletRequest request, ServletResponse response, FilterChain chain)</span> <span class="keyword">throws</span> IOException, ServletException</span>;</span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">destroy</span><span class="params">()</span></span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>这个Filter接口的定义非常简单，相当于责任链模型中的Handler抽象角色，那么它是如何形成一条责任链的呢？看另外一个类，其实在doFilter() 方法的最后一个参数我们已经看到其类型是FilterChain类，来看它的源码：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">FilterChain</span> </span>&#123;</span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">dofilter</span><span class="params">(ServletRequest request, ServletResponse response)</span> <span class="keyword">throws</span> IoException, ServletException</span></span><br><span class="line"><span class="function">&#125;</span></span><br></pre></td></tr></table></figure>


<p>FilterChain类中也只定义了一个doFilter()方法，通过调用它串联成一个责任链，实际上J2EE只是定义了一个规范，具体处理逻辑是由使用者自己来实现。</p>
<p>Netty中非常经典的串行化处理Pipeline就采用了责任链设计模式。它底层采用双向链表的数据结构，将链上的各个处理器串联起来。客户端每一个请求的到来，Netty都认为Pipeline中的所有的处理器都有机会处理它。因此，对于入栈的请求全部从头节点开始往后传播，一直传播到尾节点才会把消息释放掉。</p>
<p>Netty的责任处理器接口ChannelHandler</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">ChannelHandler</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">...</span><br><span class="line">    <span class="comment">//当handler被添加到真实的上下文中，并且准备处理事件时被调用 handler被添加进去的回调</span></span><br><span class="line">  <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Gets called after the &#123;<span class="doctag">@link</span> ChannelHandler&#125; was added to the actual context and it's ready to handle events.</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">void</span> <span class="title">handlerAdded</span><span class="params">(ChannelHandlerContext ctx)</span> <span class="keyword">throws</span> Exception</span>;</span><br><span class="line"></span><br><span class="line">    <span class="comment">// handler被移出的后的回调</span></span><br><span class="line">      <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Gets called after the &#123;<span class="doctag">@link</span> ChannelHandler&#125; was removed from the actual context and it doesn't handle events</span></span><br><span class="line"><span class="comment">     * anymore.</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">void</span> <span class="title">handlerRemoved</span><span class="params">(ChannelHandlerContext ctx)</span> <span class="keyword">throws</span> Exception</span>;</span><br><span class="line">  ...</span><br><span class="line"> &#125;</span><br></pre></td></tr></table></figure>



<p>Netty对责任处理接口做了更细粒度的划分，处理器被分成了两种，一种是入栈处理器ChannellnboundHandler，另一种是出栈处理器ChannelOutboundHandler，这两个接口都继承自ChannelHandler。而所有的处理器最终都在添加在Pipeline上。所以，添加删除责任处理器的接口的行为在Netty的ChannelPipeline中的进行了规定。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">ChannelPipeline</span></span></span><br><span class="line"><span class="class">        <span class="keyword">extends</span> <span class="title">ChannelInboundInvoker</span>, <span class="title">ChannelOutboundInvoker</span>, <span class="title">Iterable</span>&lt;<span class="title">Entry</span>&lt;<span class="title">String</span>, <span class="title">ChannelHandler</span>&gt;&gt; </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addFirst</span><span class="params">(String name, ChannelHandler handler)</span></span>;</span><br><span class="line">  </span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addFirst</span><span class="params">(EventExecutorGroup group, String name, ChannelHandler handler)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addLast</span><span class="params">(String name, ChannelHandler handler)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addLast</span><span class="params">(EventExecutorGroup group, String name, ChannelHandler handler)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addBefore</span><span class="params">(String baseName, String name, ChannelHandler handler)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addBefore</span><span class="params">(EventExecutorGroup group, String baseName, String name, ChannelHandler handler)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addAfter</span><span class="params">(String baseName, String name, ChannelHandler handler)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addAfter</span><span class="params">(EventExecutorGroup group, String baseName, String name, ChannelHandler handler)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addFirst</span><span class="params">(ChannelHandler... handlers)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addFirst</span><span class="params">(EventExecutorGroup group, ChannelHandler... handlers)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addLast</span><span class="params">(ChannelHandler... handlers)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">addLast</span><span class="params">(EventExecutorGroup group, ChannelHandler... handlers)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelPipeline <span class="title">remove</span><span class="params">(ChannelHandler handler)</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="function">ChannelHandler <span class="title">remove</span><span class="params">(String name)</span></span>;</span><br><span class="line">  ...</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>在默认的实现类中将所有的Handler都串成了一个链表。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">DefaultchannelPipeline</span> <span class="keyword">implements</span> <span class="title">ChannelPipeline</span> </span>&#123;</span><br><span class="line"><span class="keyword">static</span> <span class="keyword">final</span> Internallogger logger = InternalloggerFactory.getInstance(DefaultchannelPipeline<span class="class">.<span class="keyword">class</span>)</span>; </span><br><span class="line">  ...</span><br><span class="line">  <span class="keyword">final</span> AbstractchannelHandlerContext head;</span><br><span class="line">  <span class="keyword">final</span> AbstractchannelHandlerContext tail;</span><br><span class="line">  ...</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>在Pipeline中的任意一个节点，只要我们不手动的往下传播下去，这个事件就会终止传播在当前节点。对于入栈数据，默认会传递到尾节点进行回收。如果我们不进行下一步传播，事件就会终止在当前节点。对于出栈数据把数据写会客户端也意味着事件的终止。</p>
<h2 id="责任链模式的优缺点"><a href="#责任链模式的优缺点" class="headerlink" title="责任链模式的优缺点"></a>责任链模式的优缺点</h2><p>优点：</p>
<p>1、将请求与处理解耦；</p>
<p>2、请求处理者（节点对象）只需关注自己感兴趣的请求进行处理即可，对于不感兴趣的请求，直接转发给下一级节点对象；</p>
<p>3、具备链式传递处理请求功能，请求发送者无需知晓链路结构，只需等待请求处理结果； </p>
<p>4、链路结构灵活，可以通过改变链路结构动态地新增或删减责任；</p>
<p>5、易于扩展新的请求处理类（节点），符合开闭原则。</p>
<p>缺点：</p>
<p>1、责任链太长或者处理时间过长，会影响整体性能；</p>
<p>2、如果节点对象存在循环引用时，会造成死循环，导致系统崩溃；</p>
 
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  !function (e, t, a) {
    var initCopyCode = function(){
      var copyHtml = '';
      copyHtml += '<button class="btn-copy" data-clipboard-snippet="">';
      copyHtml += '<i class="ri-file-copy-2-line"></i><span>COPY</span>';
      copyHtml += '</button>';
      $(".highlight .code pre").before(copyHtml);
      $(".article pre code").before(copyHtml);
      var clipboard = new ClipboardJS('.btn-copy', {
        target: function(trigger) {
          return trigger.nextElementSibling;
        }
      });
      clipboard.on('success', function(e) {
        let $btn = $(e.trigger);
        $btn.addClass('copied');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-checkbox-circle-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPIED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-checkbox-circle-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
      clipboard.on('error', function(e) {
        e.clearSelection();
        let $btn = $(e.trigger);
        $btn.addClass('copy-failed');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-time-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPY FAILED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-time-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
    }
    initCopyCode();
  }(window, document);
</script>


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